Measurement of military helmet and head-mounted display (HMD) visor optical properties

This paper examines the light transmission, absorption, reflection, and scattering characteristics of military helmet visors used for see-through helmet-mounted displays (HMDs). HMDs used for the within-visual-range (WVR) counter-air mission normally use the inner surface of the helmet visor to reflect the HMD image to the pilot's eye. This approach is popular because it minimizes any optical structures that interfere with the pilot's vision, while also maximizing see-through to the ambient scene. In most cases, a reflective coating, which increases the cost of the helmet visor significantly, must be applied to the inner surfaces in order to achieve enough contrast between the HMD image and the external light passing through the visor. Recently, with the development of high luminance miniature cathode-ray-tubes (CRTs), it has been possible to eliminate the reflective coatings on neutral density helmet visors having a see-through range of 13-35%. This paper examines the light management properties of both types of visors. The paper stresses measurement techniques that produce repeatable results and what these results might imply about visual performance under operational lighting conditions.